Instruments for measuring and displaying speed or velocity of a marine vessel through water have utilized a wide variety of sensors (See, for example, U.S. Pat. No. 4,206,637--paddlewheel interrupts photodetector; U.S. Pat. Nos. 3,531,988; 3,496,770; 3,457,782--paddlewheel with magnet on each paddle or paddle made of magnetic field permeable material which is sensed by electromagnetic pick-up coil; and U.S. Pat. No. 3,706,224--paddlewheel with magnet on each paddle which interrupts current flow in a circuit).
Similarly, numerous instruments exist for providing sonic signals for depth sounding or fish locating. (See, for example, U.S. Pat. No. 4,110,727 and various references cited therein.)
It has also been found helpful for marine navigation and fish finding to measure water temperature by various means.
For the most part, prior art speedometers are mounted through the hull of the vessel requiring the drilling of a large hole in the hull and the attendant sealing problems (See the knotmeter of U.S. Pat. No. 3,531,988). Additionally, with the exception of the transom mounted speedometer, it is customary to completely enclose the paddlewheel speed sensor except for a small portion in which the paddle extends into the water (See U.S. Pat. No. 3,457,782, for example). In such "closed cavity" devices, the speed sensor becomes non-linear at higher speeds of, for example, in excess of 35 miles per hour.
U.S. Pat. No. 4,555,938 discloses a marine device which combines in one integral unit the functions of speed sensing, water temperature sensing and depth or fish sounding. In the apparatus of U.S. Pat. No. 4,555,938, a sensor housing is provided for enclosing (1) a sonic transducer assembly, (2) a temperature sensor comprising a thermistor, and (3) a solid state electromagnetic sensor comprising a Hall-effect device. The housing consists of a top end wall and four side walls. The forward wall is angularly inclined from top to bottom at an angle of about 20 degrees to conform to the inclination of the transoms of some vessels and to enable mounting the housing on such transoms. Support members comprising planar struts are fixedly attached to the aft wall opposite the inclined wall. An impeller, or paddlewheel, is rotatably supported from the struts such that only a portion of the paddles extends below the struts.
The paddlewheel, or impeller, is formed of amorphous magnetic material and the blades are magnetized to provide alternate North-South poles. As the marine vessel moves through water, the paddlewheel rotates at a rate proportional to the velocity of the vessel. A Hall-effect device mounted inside the housing adjacent the support members senses the change in electromagnetic field, as the polarized rotating paddlewheels pass by, and generates an electrical signal, the frequency of which is proportional to vessel speed.
A sonic transducer is centrally located inside the housing and is provided with a metallic coaxial shield enclosure between it and the Hall-effect speed sensing device to prevent reciprocal interference between the signals generated by the Hall-effect device and the sonic transducer. The metallic shield is also interposed between the sonic transducer and the wall mounted temperature sensor, thereby similarly reducing mutual interference between the two devices. Electrical leads running from the sonic transducer, Hall-effect device, and thermal sensor are coupled to appropriate interface circuitry and display devices on the vessel over a shielded cable through an opening in the end, or top, wall of the sensor housing. This apparatus, for the most part, has fulfilled the needs of the industry. Nevertheless, because of its integrated construction, certain problems in use have arisen, thereby creating a need for improvement.